The effectiveness of grate coolers for burned material, for example for cooling cement clinker, depends on the evenness with which the grate is covered by the material to be cooled. If the thickness of the bed of material is uneven, the cooling air driven from below through the grate will mainly flow through those regions of the bed of material which have a smaller thickness and consequently a lower flow resistance. The regions in which the thickness of the bed of material to be cooled is greater will possibly be cooled inadequately or a greater cooling input has to be applied in order to achieve adequate cooling even in those regions of the bed of material where the throughflow is less pronounced. Since the burned material to be cooled is, as a rule, not discharged completely uniformly from the upstream kiln into the initial region of the cooler, a uneven grate covering has to be expected.
To avoid this disadvantage, it is known to measure the height of the layer of material to be cooled in the initial section of the grate and to control the conveying speed of the grate in such a way that as even a layer height as possible is achieved. The layer height is determined, for example, by means of gamma-radiation meters (U.S. Pat. No. 3,064,357; U.S. Pat. No. 3,236,358; "ZEMENT-KALK-GIPS" 1967, 152-156) or via the weight of the grate covering (DE-A-195 41 455) or via special sensors (U.S. Pat. No. 2,055,941). Furthermore, it has been proposed to control the conveying speed of the cooling grate as a function of the flow resistance in the initial region of the same (U.S. Pat. No. 2,084,976; Patent Abstracts of Japan 05319877; "ZEMENT-KALK-GIPS" 1974, 559-564). This also applies to another known case (DE-A-23 27 903) in which the flow resistance calculated from the pressure below the grate is included in the control of the grate speed. In addition, the clinker throughput and the quantity of the raw meal are measured and included in the control of the feed speed in order to adapt the cooling-air quantity available as secondary air to the fluctuating process conditions of the rotary tubular kiln and to permit optimum heat recovery. The height of the bed of material to be cooled is not measured; it also cannot be derived from the quantity of raw meal or from the clinker throughput on account of the non-uniformity with which the cooling gas passes out of the kiln into the cooler. Finally, it is known to control the conveying speed as a function of the temperature of the cooler exhaust air (U.S. Pat. No. 2,031,047) or of the grate-plate temperature (U.S. Pat. No. 3,208,741) regardless of the actual height of the layer of material to be cooled. In periods of low accumulation of the material to be cooled, which at constant conveying speed would result in regions where the grate covering is slight, the conveying speed of the grate is reduced; the procedure is reversed in periods of increased accumulation of the material to be cooled. However, it has been found that the improvement which can be achieved in this way is very limited. This is due to the fact that the flow resistance depends not only on the thickness of the bed of material to be cooled but also on its grain size distribution, which likewise may very with fluctuations in the kiln operation. The coarse the clinker grain is, the lower the flow resistance is at the same height of the bed of material to be cooled. If, for example, coarse kiln residue fragments accumulate, the measured flow resistance is comparatively small, which, during control as a function of throughflow, leads to a reduction in the conveying speed of the cooling grate with the risk of overfilling of the grate.